Abstract
Abstract. A flash-flood event hit the northeastern part of Mallorca on 9 October 2018, causing 13 casualties. Mallorca is prone to catastrophic flash floods acting on a scenario of deep landscape transformation caused by Mediterranean tourist resorts. As global change may exacerbate devastating flash floods, analyses of catastrophic events are crucial to support effective prevention and mitigation measures. Field-based remote-sensing and modelling techniques were used in this study to evaluate rainfall–runoff processes at the catchment scale linked to hydrological modelling. Continuous streamflow monitoring data revealed a peak discharge of 442 m3 s−1 with an unprecedented runoff response. This exceptional behaviour triggered the natural disaster as a combination of heavy rainfall (249 mm in 10 h), karstic features and land cover disturbances in the Begura de Salma River catchment (23 km2). Topography-based connectivity indices and geomorphic change detection were used as rapid post-catastrophe decision-making tools, playing a key role during the rescue search. These hydrogeomorphological precision techniques were combined with the Copernicus Emergency Management Service and “ground-based” damage assessment, which showed very accurately the damage-driving factors in the village of Sant Llorenç des Cardassar. The main challenges in the future are to readapt hydrological modelling to global change scenarios, implement an early flash-flood warning system and take adaptive and resilient measures on the catchment scale.
Highlights
Flash floods are related to high-intensity precipitation, mainly convective in origin and with restricted spatiotemporal occurrence
In addition to the ability of high-resolution digital terrain models (HR-DTMs) to improve sediment connectivity as a powerful tool to determine preferential flow paths and deposition areas, the present study evaluated the landform signatures of the event by using unmanned aerial vehicles (UAVs) as a rapid-response tool for post-catastrophe search and rescue tasks along the whole downstream section of the Ca n’Amer River from the village of Sant Llorenç des Cardassar, in order to measure effectively the sediment deposits generated by the flash flood and to locate and quantify the most important deposition areas downstream from where the person was last seen
The hydrogeomorphological analysis and damage assessment developed in this paper has provided a comprehensive understanding of the Sant Llorenç des Cardassar flashflood event of 9 October 2018 by means of an integrated approach with a meteorological, hydrological, geomorphological, damage and risk data analysis
Summary
Flash floods are related to high-intensity precipitation, mainly convective in origin and with restricted spatiotemporal occurrence. For this reason, they usually strike basins < 1000 km with response times of a few hours or less. In Europe, 40 % of flood-related casualties in the period 1950–2006 are due to flash floods (Barredo, 2007). Catastrophic flash floods are much more frequent in some parts of the Mediterranean region than in the rest of Europe, due to the interaction between geomorphology, climate, vegetation and the warm sea surface (Cassola et al, 2016), all combining to create a flood-prone environment. The abrupt reliefs surrounding the Mediterranean Sea are very close to the coastline, shaping small and torrential catchments where the convergence of low-level atmospheric flows and the uplift of warm wet air masses drifting from the Mediterranean Sea to the coast generates heavy downpours in very short time spans (Gaume et al, 2009)
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